It is well known that degradation of S/N ratio can be reduced largely by transmitting and processing various signals in the form of digital signal. From this point of view, even in the case of an image sensor, attempts have been made to convert a video signal into a digital signal in a stage as early as possible so that various processings of digital signals are carried out in the succeeding stages.
In a prior art image sensor, however, even in a case that a signal obtained by photoelectric conversion is converted into a digital signal in an earlier stage, various analog information obtained from picture elements of a solid stage image sensor element is once derived from the elements as time sequential analog information, and then the analog information is converted into a digital signal through an analog-to-digital (A/D) converter. That is, as shown in FIG. 1A, an analog signal derived from an image pickup tube 2A is applied through a buffer amplifier 4A to an A/D converter 6A to obtain a digital signal. Alternatively, as shown in FIG. 1B, a signal derived from a solid state image sensor element 2B is applied through a buffer amplifier 4B to an A/D converter 6B to obtain a digital signal.
Video signals are processed in various manners, so that the processing as shown in FIG. 1A or 1B in which the video signal is processed after the signal is converted into the digital signal has a merit, but with the processing as shown in FIG. 1A or 1B, the processing of an analog signal in an image pickup element prior to the A/D conversion is carried out in a conventional manner and the problems there are left unsolved. As a result, there is a disadvantage that a high quality image cannot be picked up, since an S/N ratio cannot be improved to a satisfactory degree due to superposition of various noise.